My problem with the ending is that it's needlessly realistic. It's true that relationships don't always work out, so it makes sense that Robin and Barney eventually split up. It's also true that some people die tragically young of disease, so of course that could happen to Tracy. However, HIMYM is a show that isn't realistic. Ted's romantic gestures usually work, the group of five are always great friends, and many things that happen in the show border on magical realism (things like the cockamouse, every member of the group having a doppelganger, Ted being able to make it rain from a storm that was supposed to pass over NY, etc). HIMYM was a very feel-good show from the start. Sure there were sad moments, but those always brought the group closer together and kept it from being a completely over-romantic slimefest.
Tracy dying isn't feel-good though, it's random and it sucks, we did not need that in a tv-show. Barney and Robin getting divorced also feels like a middle finger, when multiple seasons have been dedicated to Barney growing as a person and allowing himself to be with someone in a meaningful way, as well as Robin growing as a person and deciding to give her lovelife a shot as well as her carreer.
The ending to HIMYM could have worked in some shows, but it definitely didn't work in this show.